Chandra' s observation in March of
2001 of the neutron star KS 1731-260 (pale blue dot
just above the middle of the image) showed that it is a
remarkably 'cool' 3 million degrees Celsius.
This low temperature was surprising because in the
period 1988 to 2000 the neutron star was shining
brightly in X-rays due to the heavy bombardment of gas
from a companion star, not visible here. A neutron star
temperature of 10 million degrees or more was
expected.

A possible explanation for the relatively low
temperature of KS 1731-260 in its present quiet state
is that it was in a deep freeze for a thousand years
before 1988 and took 12 years of heating just to get to
the temperature it is today. If so, it may represent a
new type of neutron star system that stops accreting
gas for a long period of time, and there could be
hundreds of undetected, cold neutron stars in our
Galaxy.